why freind function takes more parameter than normal member function in c++?

why freind function takes more parameter than normal member
function in c++?

Question Posted / samanway90

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why freind function takes more parameter than normal member function in c++?..

Answer / jeremiah

A friend function needs to know the object on which it must
operate. Class member functions have an implicit "this"
pointer which define the object on which it must operate.

Example:
---------------------------------------------------------
class Number;

// A function that adds two numbers.
// This function must be a friend of the Number class
// because it operates on a private member variable of the
// Number class.
Number Add( const Number& lhs, const Number& rhs )
{
Number result;
result = lhs.m_value + rhs.m_value;
return result;
}

// The Number class
class Number
{
public:
// Constructor - Default to 0.
explicit Number( int value = 0 )
: m_value( value )
{}

// Copy constructor
Number( const Number& copy )
: m_value( copy.m_value )
{}

// Add this number with another and return the result.
// This operator only needs 1 argument because the
// Left-hand side is assumed to be the class itself.
Number operator+( const Number& rhs ) const
{
Number result;
result.m_value = m_value + rhs.m_value;
return result;
}

private:
// Allow the Add function to access the private members
// of this class by making it a friend function.
friend Number Add( const Number&, const Number& );
int m_value;
};
-----------------------------------------------------------

As you can see from the example, the "Add" function takes
two arguments (a left-hand side operator, and a right-hand
side operator), whereas the Number::operator+ member
function only needs to know the right-hand side operator
because the left-hand side operator is assumed to be the
class instance itself.

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